1. Field of the Invention
The present invention relates to an LED driving apparatus. More particularly, the present invention relates to an LED driving apparatus for driving LEDs (light emitting diodes) with a PWM (pulse width modulation) signal.
2. Description of the Related Art
Generally, a plurality of LEDs, which are provided as an array for three primary colors red, green and blue, is used for a backlight unit of an LCD (liquid crystal display).
A conventional LED driving apparatus may be constructed as a circuit shown in FIG. 1. In an LED driving apparatus 1 of FIG. 1, when a switching FET (field effect transistor) 5 is turned on, current Io supplied from a power supply 15 flows through a plurality of LEDs 9 and an inductor 11. If the switching FET 5 is turned off, current flows through the inductor 11, a diode 7 and the LED 9. The LED driving apparatus may further include a capacitor 13 to decrease electromagnetic interference or current stress on the LEDs 9.
In more detail, a control block 3 controls flow of current through the LEDs 9 based on a PWM signal having a predetermined duty cycle. When the current flowing through the LEDs 9 reaches a predetermined peak value Iref, the control block 3 controls the current flowing through the LEDs 9 to be cut off. Therefore, the amount of current flowing through the LEDs is adjusted to control the amount of light emitted from the LEDs 9.
For example, if a logic high PWM signal is input, the control block 3 controls the switching FET 5 to be turned on, thereby increasing the amount of current flowing through the LEDs 9. When the current flowing through the LEDs 9 reaches the peak value Iref, the control block 3 controls the switching FET 5 to be turned off, thereby decreasing the amount of current flowing through the LEDs 9. After a predetermined time elapses and the PWM signal goes into logic high, the control block 3 controls the switching FET 5 to be turned on, thereby increasing the amount of current flowing through the LEDs 9. These operations are repeatedly performed.
However, when the control block 3 transmits an on signal or an off signal to the switching FET 5, there may be a delay. Such a delay may cause a problem if the LEDs 9 are short-circuited, for example, when the LEDs 9 reach an end of life or when any defect occurs in the LEDs 9.
For example, in a state where the amount of current, after reaching the peak value Iref, is not sufficiently decreased, if the switching FET 5 is allowed to be turned on by the PWM signal, the amount of current flowing through the LEDs 9 increases again. Then, the amount of current flowing through the LEDs 9 reaches the peak value Iref, and then the control block 3 transmits the off signal to the switching FET 5.
However, at this time, if there is a delay in transmitting the off signal to the switching FET 5, a suitable timing capable of controlling the current amount flowing at the LED 9 is missed, and accordingly, the amount of current flowing through the LEDs 9 exceeds the peak value Iref while the switching FET 5 remains turned on. The control block 3 cannot adequately control the amount of current overflowing through the LEDs 9. Ultimately, the amount of current flowing through the LEDs 9 continues to increase, potentially damaging the circuit.
Accordingly, there is a need for an improved apparatus and method for driving an LED.